A floating roof storage tank generally has a solid cylindrical outer wall covered by a solid dome-shaped roof. A floating roof is held within the volume defined by the outer wall and roof. The floating roof extends over the liquid contents held within the volume, and forms a vapor seal around the internal circumference of the cylindrical wall. The height within the tank at which the floating roof is positioned varies according to the amount of liquid being stored within the tank and the rate at which the liquid is pumped out of the tank.
Floating roof storage tanks generally are used to store flammable liquids, such as gasoline. A vapor space is formed inside the tank, between the floating roof covering the gasoline and the outer tank roof. Vapors emitted from the internal volume of the storage tank are collected in the vapor space to prevent significant amounts of vapor from being expelled to the atmosphere outside the tank. Environmental protection regulations restrict the amount of hydrocarbon vapors that may be released to the atmosphere. Air quality may be impacted adversely if hydrocarbon vapor emissions exceed permitted amounts.
Manual access to the internal volume of the tank is provided within the vapor space. One or more access ports or doorways are formed in the roof or outer wall of the tank. Persons may enter the vapor space through such access port(s) to service the tank, the floating roof and any equipment housed inside the tank.
Special precautions are taken when introducing equipment into the internal volume of the floating roof storage tank when the tank volume holds a flammable liquid, such as gasoline. Measures are taken to prevent sparks and associated combustion of flammable vapors. Measures are also taken to minimize the amount of time the seal between the flammable liquid and the vapor space above the floating roof is broken to prevent substantial amounts of flammable vapors from being emitted into the vapor space.
The floating roof frequently is provided with an opening or hatchway for access to the internal tank volume under the floating roof. That hatchway is covered with a hatch door that is sealed when closed to prevent unintended release of vapors. The hatch door also may be provided with grounding means to minimize the possibility of a spark.
Floating roof storage tanks are inspected at regular intervals to locate cracks, corrosion or other defects that might lead to tank failure. Environmental protection regulations specify the frequency and recommended procedures for inspecting tanks for structural integrity. One method involves introducing a remote controlled submersible vehicle into the tank while the tank remains in service. For example, U.S. Pat. No. 5,205,174 discloses a scavenger submersible vehicle that inspects the internal surfaces of a liquid filled tank using a video camera and/or ultrasound pulses. One or more umbilical hose(s) and wire harness(es) connect the remote-controlled vehicle to its power source and air or vacuum source, and further provide a link for transmitting navigation and inspection data from the vehicle to one or more computers located outside the tank.
While the vehicle readily may pass through the hatchway opening formed in a floating roof of a floating roof storage tank, the umbilical hose(s) and wire harness(es) extend through that opening and prevent re-positioning of the existing hatch cover over the hatchway opening. The submersible vehicle takes substantial time to traverse the inner surfaces of the tank to complete its inspection, and during that time, vapors from the tank can escape into the vapor space through the open hatchway. Thus, the industry seeks an improved hatch cover that can seal the hatchway opening while the inspection vehicle is deployed within the tank.
U.S. Pat. No. 1,592,524 discloses a floating roof tank with a cover plate that is pivotally secured to the upper floating wall. The cover plate has a recess to permit a pipe to pass therethrough. The tank is intended for storing oil, and no means are provided for minimizing vapor emissions. See also U.S. Pat. No. 945,753 showing a trap door with an opening for a pipe to pass therethrough.